1. Field of the Invention
The invention relates generally to electrical control systems, and more particularly to systems and methods implemented in variable speed drives for electric submsersible pumps to determine when a pump can be restarted.
2. Related Art
Crude oil is typically produced by drilling wells into oil reservoirs and then pumping the oil out of the reservoirs through the wells. Often, the oil is pumped out of the wells using electric submersible pumps. Electrical power is provided to electrical drive systems at the surface of the wells and these drive systems provide the required electrical power to the downhole pumps.
While downhole pumps are designed to operate continuously, they are subject to interruptions that can result from a number of different causes. For example, changes in well conditions (e.g., the appearance of gas in an oil well) may cause the pump to stop operating. Interruptions or variations in the power supplied to a pump's drive system may also cause operation of the pump to be interrupted. Even if these interruptions in the operation of the pump are relatively short, they may nevertheless be very disruptive, particularly when the pumps are submersible pumps operated in deep wells.
These interruptions may be very disruptive because submersible pumps, which must fit in a well and must therefore be long and narrow, have very little inertia. Consequently, when there is a change in conditions which causes an interruption, these pumps slow down or stop very quickly in comparison to pumps which have more inertia, such as surface pumps. The deceleration of the pump is even more pronounced in deep wells due to the large fluid column above the pump. Normally, when the operation of the pump is interrupted for longer than about half a second, the pump will have begun to spin in reverse.
Typically, there is some speed below which the pressure produced by the pump is insufficient to support the column of fluid. When the rotation of the pump falls below this speed, the fluid starts to fall back through the well and through the pump, dramatically increasing the torque required to resume forward rotation of the pump. While it is possible to match the speed of the pump motor, slow its reverse spin and start it spinning forward again, this often requires a great deal of torque. The torque that can be generated by the pump system may be limited by such factors as the output of the drive for the pump motor, the impedance of the cable carrying the power downhole, etc., so restarting the pump motor may require more torque than the system can generate. It is therefore typically necessary to stop the pump and wait for the column of fluid to drain from the well before the pump can be restarted. The time required for the fluid column to drain back into the formation may take a few minutes in some cases, while in other cases it may take more than an hour.
Normally, when it is necessary to restart a pump, an operator waits for a predetermined period and then restarts the pump. The wait period is typically determined by adding the amount of time necessary for the fluid to completely drain from the well and a safety margin (for example, an additional 25%.) Because each well may normally produce hundreds or even thousands of barrels of oil in a day, the cost associated with the delay between the pump stopping and being restarted can be very high. There is therefore a need to minimize the delay between the time the pump stops and the time the pump is restarted.